Converting Quaternions to Euler in the Graph Editor
Select the camera object, (still in quaternion rotation mode) copy script, paste into text editor, run script.  It will create a set of rotation_euler fcurves matching the quaternion rotation.  If all goes well manually change the rotation mode and clear the quaternion keyframes.

import bpy
from math import tan, atan
context = bpy.context
scene = context.scene
order = 'XYZ' # euler rotation order desired
cam_obj = context.active_object
action = cam_obj.animation_data.action

frames = []
fcurves = []

for index in [0, 1, 2, 4]:
    # search for rotation euler kfps
    fc = action.fcurves.find("rotation_quaternion", index=index)
    # make a list of frames that have quaternion rot keyframes
    if fc:
        frames.extend([kfp.co[0] for kfp in fc.keyframe_points])
        fcurves.append(fc)
# run thru and keyframe in 

for f in set(frames):
    scene.frame_set(f)
    cam_obj.rotation_euler = cam_obj.matrix_world.to_euler(order)
    #cam_obj.rotation_euler = [atan(tan(d)) for d in cam_obj.rotation_euler]
    cam_obj.keyframe_insert("rotation_euler", frame=f)